You are answering a USMLE-style medical question. Question: A 40-year-old male presents to your office complaining that he is too weak to climb stairs or brush his hair. He denies any headaches or change in vision. A muscle biopsy reveals CD8+ lymphocyte infiltration in the endomysium. Which of the following is the most likely diagnosis? Options: A) Systemic lupus erythematosus B) Polymyalgia rheumatica C) Polymyositis D) Dermatomyositis Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.